EPR Study of Polyaniline Synthesized Enzymatically in the Presence

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EPR Study of Polyaniline Synthesized Enzymatically in the Presence of Submicrometer-Sized AOT Vesicles Boris Rakvin, Dejana Cari#, Mladen Andreis, Katja Junker, and Peter Walde J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp411204b • Publication Date (Web): 31 Jan 2014 Downloaded from http://pubs.acs.org on February 4, 2014

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The Journal of Physical Chemistry

EPR Study of Polyaniline Synthesized Enzymatically in the Presence of Submicrometersized AOT Vesicles Boris Rakvin,*† Dejana Carić,† Mladen Andreis,† Katja Junker‡ and Peter Walde‡ †

Ruñer Bošković Institute, Division of Physical Chemistry, Bijenička c. 54, 10000 Zagreb,

Croatia ‡

Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, CH-8093 Zürich, Switzerland

ACS Paragon Plus Environment

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ABSTRACT

EPR spectroscopy was used to examine the magnetic properties of two enzymatically synthesized polyaniline (PANI) samples obtained in the presence of submicrometer-sized vesicles formed from sodium bis(2-ethylhexyl)sulphosuccinate (AOT) as templates. PANIHRPC-AOT was synthesized with horseradish peroxidase isoenzyme C (HRPC) and hydrogen peroxide (H2O2) as oxidant while PANI-TvL-AOT was prepared with Trametes versicolor laccase (TvL) and dioxygen (O2) as oxidant. A commercial conductive sample of the emeraldine salt form of polyaniline (PANI-ES) was also used for comparison in order to correlate the experimental data obtained for PANI-HRPC-AOT and PANI-TvL-AOT with the properties of the well-characterized PANI-ES. It was shown that a model based on the concept of correlated polaronic bands could be applied for the interpretation of the EPR spectra of all three examined samples, although PANI-HRPC-AOT and PANI-TvL-AOT were significantly less conductive than PANI-ES. The magnetic properties of the PANI samples could be related to their conductivities, whereby a low conductivity was ascribed to decreased interchain spin interactions which were detectable from a splitting of the triplet spectrum at low temperatures (5 K - 10 K). The obtained effective distance between the polyaniline chains is larger for enzymatically synthesized PANI than for PANI-ES, most likely mainly due to the presence of AOT which could not be removed completely during the workup. AOT influences the chain conformation and the average chain-chain distance. KEYWORDS Conductive polymers; PANI; Polaron pairs; Polaron bands; Spin exchange interaction


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1. INTRODUCTION Magnetic properties of the conductive polymer polyaniline (PANI) and its derivatives have been studied extensively during the past decades. 1 These properties are relevant for a better understanding of the nature of the charge carriers in the polymer structure. It can be noted that most of the PANI samples studied by EPR showed a nearly linear temperature dependence of the magnetic susceptibility (χ) multiplied by temperature, χT vs. T. This behaviour is usually described by taking into account two types of contributions, (i) a temperature dependent Curie paramagnetism and (ii) a temperature independent Pauli type paramagnetism. The Pauli susceptibility (χp) is related to the spin delocalisation and is directly proportional to the density of states at Fermi level (N(EF)). Usually, this is attributed to the conductive so called threedimensional ‘’metallic’’ state of PANI. 1b However, it was noted that such simple procedure yields rather larger values of χp for various polymers with low dc conductivity and also that some of the χT vs. T plots exhibit deviation from linearity especially at low and high temperatures. The magnetic properties of PANI systems have been studied by Kahol and collaborators 1e, f and a nearly linear χT dependence on T was attributed to disorder-induced localized polaron pairs (pairs of radical cations with S=1/2). It was shown that the averaging of the susceptibility of antiferromagnetically coupled pairs described by the Bowers-Bleaney formula over a wide distribution of exchange interaction (Ji) exhibits linear dependence. For the presence of an additional non linear dependency of χT on T one could expect some dominant J within the distribution of Ji, leading to some specific or dominating exchange interactions present in the monitored sample. Indeed, this type of behavior was also previously detected 2 and a model with one or two exchange interactions (denoted by Cik et al.2 as activation energy, ∆E)


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was employed. Recently, the nonlinear dependence of χT vs. T was described with a model which takes into account ferromagnetically and antiferromagnetically correlated polaron bands. 3 In the present work we employed EPR spectroscopy to examine the magnetic properties of two enzymatically synthesized PANI samples. Both were obtained by using submicrometer-sized vesicles formed from sodium bis(2-ethylhexyl)sulphosuccinate (AOT) as templates for the polymerization.4 The first sample, PANI-HRPC-AOT, was synthesized with horseradish peroxidase isoenzyme C (HRPC) and hydrogen peroxide (H2O2) as oxidant and the second, PANI-TvL-AOT, was obtained with Trametes versicolor laccase (TvL) and dioxygen (O2) as oxidant. In addition, a commercially available conductive sample of the emeraldine salt form of polyaniline (PANI-ES) was also used for comparison. By employing EPR spectroscopy it is expected to achieve an enhanced detection of the local paramagnetic properties together with an increase in sensitivity for the detection of the distribution of different magnetic species, if compared to SQUID measurements which yield information about the average magnetization of the examined sample only.1i Indeed, on the basis of the detailed EPR measurements presented in this paper, the observed differences in the conductivities of PANI-HRPC-AOT, PANI-TvL-AOT and PANI-ES4c became clear. The differences seem to be mainly due to the various distances between PANI chains in the samples whereby larger interchain distances, originating from the presence of AOT molecules which could not be completely removed correlate with lower conductivities. 2. EXPERIMENTAL SECTION The powder samples of PANI-HRPC-AOT,4b PANI-TvL-AOT4c and PANI-ES (from Aldrich, product 42,832-9, lot #S24379-155, average Mw>15,000 g/mol, dopant: proprietary organic


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sulfonic acid) (see4b,

c

for details) were placed and sealed in simple quartz EPR tubes at

atmosphere pressure without any additional removing of possible excess of oxygen from the samples. EPR spectra were recorded with a Bruker ELEXIS 580 spectrometer operating at Xband frequencies. The spectrometer was equipped with a standard cryostat with capability of temperature variation in a wide temperature interval (5 K - 300 K). All EPR spectra were obtained with minimum intensity of microwave power (~1 µW). Concentrations of the paramagnetic centers as well as g-values were measured by employing standard procedure with appropriate EPR cavity (double rectangular X-band resonator) and strong pitch Bruker standard at room temperature. 3. RESULTS AND DISCUSSION 3.1. EPR characteristic parameters of the PANI samples. The expected high concentration of paramagnetic centers (PCs) in the investigated PANI samples led to an EPR signal with large intensity if recorded with an ordinary X-Band EPR spectrometer. Thus, even at the minimum microwave power employed, a nonsymmetrical singlet type of spectrum (in absorption mode at first harmonic of modulation) was easily recorded. It was clearly evident that the typical spectrum deviated from usual Gaussian or Lorentzian lineshape. The asymmetry of the spectrum for the PCs in conducting PANI samples is expected to be of the Dyson lineshape due to contribution of the portion of dispersion mode besides the dominantly detected absorption mode. One possibility to compare the examined samples is to use X-band EPR spectroscopy to evaluate the characteristic parameters at room temperature. The obtained spectra are described with simple peak to peak linewidth (∆pp), ratios of peak amplitudes (a/b) expected for Dyson line shape, average g-values and spin concentrations (Table 1). The estimated parameters of the


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Table 1. EPR characteristic parameters of the PANI samples investigated at room temperature. Sample

Peak to peak linewidth (∆pp)/mT

a/b

g-value

Spin concentration/ spin/g

PANI-HRPC-AOT

0.41

1.09

2.0055

3.36 x 1020

PANI-TvL-AOT

0.28

1.21

2.0046

1.47 x 1020

PANI-ES

0.18

1.36

2.0053

1.29 x 1020

PANI-ES sample could be scaled on the basis of known literature data obtained for well characterized doped PANI emeraldine base with a similar value for the peak-to-peak linewidth at room temperature .1c According to this scaling procedure it is expected that PANI-ES with a doping level of y

0.4 (number of dopant molecules per monomer) has a conductivity, σdc,

1

S/cm. The same scaling procedure for EPR linewidths measured for PANI-TvL-AOT and PANIHRPC-AOT leads to much lower effective ‘’y’’ and about a four order of magnitude lower conductivity. Indeed, a reduced conductivity for PANI-TvL-AOT (σdc = 6.6·10-5 S/cm) and PANI-HRPC-AOT (σdc = 3.2·10-5 S/cm) samples in comparison to a commercial PANI-ES sample (σdc = 2.5 S/cm) was recently measured at room temperature.4c Moreover, for the temperature dependence of PANI-ES with such doping level, a bell-type dependence is expected with a critical temperature Tc ≈ 200 K.1c Thus, as was demonstrated earlier for the high temperature interval (80 K < T< 300 K), at X-band frequencies,1c ∆pp linearly increases with increasing temperature at T< Tc, and decreases with increasing temperature at T > Tc.1c In order to examine in more detail the properties of the PANI samples, the temperature dependence of ∆pp was investigated for PANI-ES, PANI-HRPC-AOT and for PANI-TvL-AOT. Plots of ∆pp vs. T in a wide temperature interval (5 K - 300 K) for all three PANI samples are shown in Fig. 1. It is easy to note qualitatively different temperature dependencies between the enzymatically


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synthesized PANI samples and the commercial PANI-ES sample. In the entire temperature interval, PANI-TvL-AOT and PANI-HRPC-AOT exhibit larger ∆pp than PANI-ES. On the other hand, ∆pp of PANI-ES decreases with increasing temperature in the low temperature interval (5 K - 20 K), then increases in the second temperature interval (20 K - 200 K) and finally it is nearly independent on temperature in the third temperature interval (200 K - 300 K). The weak maximum near 200 K, between the second and third interval, could be

1

∆pp (mT)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.1

1

10

100

1000

T (K)

Figure 1. Peak to peak linewidth (∆pp) of the EPR spectrum as a function of temperature for the different PANI samples analyzed: PANI-ES (circles), PANI-HRPC-AOT (triangles), and PANITvL-AOT (squares).

attributed to Tc of PANI-ES in this higher temperature interval. The data obtained by both highfrequency EPR (D-band at 140 GHz) and electron transport methods 1c, 5 show that heavily doped PANI reach maximum conductivity at Tc. The conductivity mechanism in the suggested model is dominated by 1D localization of electrons (semiconductor regime) at T ≤ Tc and 3D electron localization (metal regime) in clusters formed at higher temperatures. The increased


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conductivity could be described by Mott’s law,6 as conductivity of clusters at T ≤ Tc and decreasing conductivity processes at T ≥ Tc are influenced by scattering of charge carriers on the optical lattice phonons.7 The temperature dependency of the linewidth of the EPR spectrum of PANI-ES can be scaled qualitatively with heavily doped PANI in the high temperature interval (100 K - 300 K) and the presence of metal like clusters which are composed of strongly interacting polymer chains characterized by 3D delocalization of the conducting electrons. However, in the low temperature interval (5 K - 20 K), there is a completely opposite temperature dependency of ∆pp (Fig. 1) than in the high temperature interval, indicating that an additional examination and a more detail study to elucidate the appropriate mechanism of the spin dynamics are needed. Moreover, ∆pp of both samples PANI-TvL-AOT and PANI-HRPCAOT show the same temperature dependence in the whole monitored temperature region (5 K 300 K), while the behavior of PANI-ES is very different. In particular, no evidence for the presence of metal like clusters with 3D delocalization of the conducting electrons in the enzymatically prepared samples could be deduced by monitoring only the ∆pp parameter. In order to resolve additional characteristic parameters related to the spin dynamics, a detailed study of linewidths in the low temperature interval as well as a study of the EPR signal intensity in the whole temperature interval was undertaken, as described in the following paragraphs. 3.2. Temperature dependence of the EPR signal intensity. The EPR signal of PANI-ES is a composite spectrum of two dominant components arising from two dominant radicals (R1 and R2). R1 and R2 were detected at X-band frequencies (

9.5 GHz) and are represented in the

spectrum with an inhomogeneously broadened line in the high temperature region. In the same temperature region, the PANI-ES spectrum can be detected with higher resolution as separated spectral components by employing detection at higher EPR frequency (140 GHz) range.1c, 5


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These radicals slightly differ in effective g and A tensors due to different local sites and unpaired electron migration. It is assumed that the radical R1 represents a polaron state localized in the polymer chain while the radical R2 is a polaron that diffuses along the polymer chain. The relative mutual concentration of these radicals is temperature and doping-level dependent. The EPR intensity decreases sharply with increasing temperature for all three samples. The intensity of the EPR signal is also proportional to the magnetic susceptibility and it is convenient to discuss and to correlate these signal intensity data in terms of magnetic susceptibility of the PANI samples. However, the temperature dependence of signal intensity plot shows very similar behavior for all three samples. Therefore, this analysis is not sensitive enough to evaluate differences between the three samples. In order to better assess differences between PANI-ES, PANI-TvL-AOT and PANI-HRPC-AOT, it is appropriate to use a plot of EPR intensity multiplied with temperature as a function of temperature. In Fig. 2, these types of plots indeed show larger differences at higher temperatures than in the low temperature interval where all samples exhibit comparable values. The plots in Fig. 2 are proportional to the well known susceptibility plot of χT vs. T which is usually employed in studies of the magnetic properties of conducting polymers. 1f, 1i, 2-3, 8 In earlier investigations of various forms of PANI, a linear dependence of χT on T in the high temperature region was observed. This behavior was explained in terms of Pauli-like contributions due to exchange-coupled polaron pairs.1f In the plots shown in Fig. 2, a clear deviation from linearity can easily be noted and a model that considers correlated polaron bands was applied to describe the obtained experimental data: 1i, 3

4 / 3 exp 4 / 3 exp ⋯

(1)


9


The Curie constants C0, C1, and C2 represent non interacting (C0) and interacting (C1 and C2) polarons, respectively. When J 0) in energy the crystalline region will show ferromagnetic or antiferromagnetic behavior, respectively. In disordered regions of the crystal, disorder will prevent complete S = 1 and S = 0 coupling and bands of coupled ferromagnetic and antiferromagnetic polarons will appear.


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The ratio between interacting and non-interacting polarons can be obtained from the evaluated constants Ci in equation (1) by employing normalized data in the monitored temperature interval. The experimental data, shown in Fig. 2, significantly deviate from a horizontal line (Curie behavior) which originated from non-interacting paramagnetic centers of localized spins. Thus, it can be assumed that variation in χT with T in Fig. 2 indicates significant contributions related to the interaction of polarons. The relation (1) was adapted to qualitatively describe spin concentrations (N, number of spins/2-rings) for all PANI samples in the whole monitored temperature region. In accord with the above observation one expects that the concentration of spins N (T) ∝ EPR Intensity x T ∝ χT as a function of temperature is described with corresponding proportional coefficients. The continuous lines in Fig. 2 represent the obtained fits to equation (1) and the corresponding proportional coefficients obtained for the best fit of the experimental data are given in Table 2. One notes that the quality of the fit is satisfactory by taking into account contributions of only two exponentially dependent terms of relation (1) which describes two pairs of antiferromagnetically correlated polarons (J > 0). In all measured PANI samples, there are contributions from two pairs of well distinguishable antiferromagnetically coupled polarons (polaron bands) and from non-interacting polarons. The percentage of each band can be easily evaluated and compared among the samples. PANI-ES

Table 2. EPR Estimated spin concentration from the EPR signal intensity and obtained fitted parameters for relation (1) and the experimental data shown in Fig. 2 for PANI-ES, PANIHRPC-AOT and PANI-TvL-AOT. The absolute accuracy for N is within one order of magnitude (Ci = Nig2µβS(S+1)/3k).


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Sample

N0 (spins/2rings) x103

N1 (spins/2rings) x103

J1 (K)

N2(spins/2rings) x103

J2 (K)

N (spins/2rings) x103 at 295 K

PANI-ES

5.6±0.4

10.8±0.4

52±4

104±8

744±24

65

PANIHRPCAOT

6.5±1.0

38.2±2.4

123±10

1050±227

1133±67

148

PANITvL-AOT

2.7±0.5

15.2±0.7

70±6

153±12

752±26

56

shows the lowest band exchange energies J1 and J2 if comparison is made to the two enzymatically prepared PANI samples. PANI-HRPC-AOT exhibits bands with the largest exchange coupling (J2 =1133 K) which indicates presence of the largest disordered region among the measured samples. 2-3 The order of magnitude for the obtained J1 and J2 values coincides with the expected values obtained by quantum-chemical calculation for aniline dimers and tetramers for singlet-triplet transitions in relative short sections of the polymer chain.9 It should be noted that the presence of ferromagnetic bands at high temperature is not resolved with the applied fitting procedure. The obtained J values represent an effective value since the EPR spectrum exhibits inhomogeneous character containing several spectral components which are more pronounced at low temperatures. A possible extrapolation of the linewidths of these components has been undertaken. The activation processes of linewith component narrowing have been employed to examine the possible presence of more localized specific exchange processes characterized by a corresponding J value. 3.3. EPR spectral intensities at low temperatures. As was mentioned above, a composite character of the EPR spectra was possible to resolve at high frequency. 1c, 5 The EPR spectrum (caused by R1 and R2 centers) of PANI measured at D-band frequencies at room temperature is


12

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composed of several components, while the same spectrum recorded at X-band frequencies is mostly a single line spectrum. However, the same sample giving a single line spectrum at Xband frequencies at high temperature yields a more complex spectrum at very low temperatures. It is important to note that it is possible to decompose the low temperature EPR spectrum into two type of lines, one low intensity line with broad Gaussian shape and one line with narrower Lorentzian shape which allowed to explain the band type model, as discussed previously 3. In the present study evidence for the existence of at least two different groups of PCs related with two different components which correspond to the cumulative EPR spectrum detected at X-band frequency was experimentally obtained by employing the method of in-phase and out-of-phase detection.10 In order to reach separation between possible components, one expects a small difference in the phase shift for the amplitude modulation signal with modulation frequency, νM = 100 kHz, in the vicinity of the 90o out of modulation phase. Fig. 3 shows typical EPR spectra for the PANI-TvL-AOT sample recorded in steps of 0.1o in the vicinity of the 90o out-of-phase modulation frequency. A dominant contribution of a broad component was detected at around 0.3o below the out-of-phase frequency and a narrow component is detected at 0.5o above the outof-phase frequency. It follows that the recorded PANI-TvL-AOT spectrum contains two components originating from two PC species with slightly different effective spin–lattice relaxation times, T1. 10b The obtained components and their proportional intensities could be used to decompose the complex in phase (0o) spectrum as shown in Fig. 3. Moreover, the


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90.5o

90.0o

89.5o

335

340

345

350

355

360

Magnetic field (mT)

b) EPR Intensity (arb. units)

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EPR Intensity (arb. units)


335

340

345

350

355

360

Magnetic field (mT)

Figure 3. a) EPR X-band spectra of PANI-TvL-AOT recorded in the vicinity of 90o (“out-ofmodulation phase” spectrum) for each 0.1o of phase at 5 K. The spectrum shows disappearing of composite signal at two different phase angles. Two signal components characterized with different splitting are denoted with dashed (red) and dotted (blue) lines. b) The EPR signal detected in phase at 0o with modulation signal decomposed into two proportional spectral components, dashed (red) and dotted (blue) as originally separated and detected in out-of-phase mode as shown in figure a).


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low intensity component exhibits doublet splitting and indeed this is expected for a powder spectrum of triplet states with comparable amounts of exchange interaction and zero field splitting contributions11 as will be also discussed below. Generally, a nonuniform disappearance of different components with increasing temperature was observed for all three PANI samples. This property was also applied through an additional and convenient way of decomposing complex spectra in order to correlate it with different PC species at low temperatures. For example, the X-band spectrum of PANI-ES at 5 K shows composite character arising from two prominent components as shown in Fig. 4. It was found that the intensity of

a) EPR Intensity (arb. units)

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b)

c)

340

342

344

346

348

350

352

354

Magnetic field (mT)

Figure 4. The solid line shown in a), represents the EPR X-band spectrum of the PANI-ES sample recorded at 5 K. The dashed spectrum, b), is that part of the spectrum a) which exhibited faster disappearing at higher temperature (7 K) than the rest of the spectrum. The spectrum, c), shown as dashed line, was obtained as a result of the subtraction of b) spectrum from a) spectrum.


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the outer part of the spectrum decreases with increasing temperature, while in the same time, the narrow singlet at the central position becomes narrower with increasing temperature. This observation supports a composite character of the spectrum which contains at least two detectable components. If the EPR spectrum obtained at higher temperature (7 K) is subtracted from the spectrum recorded at lower temperature (5 K) a temperature dependent fast decaying spectral component emerges. The obtained component exhibits the structure of a nearly symmetric “doublet” and could be used to decompose the spectrum recorded at 5 K into two components as shown in Fig. 4. Since, with increasing temperature the obtained “doublet” decreased significantly faster than the portion of the central part of the spectrum, as demonstrated in Fig. 5 for a spectrum measured at 13 K, the spectrum appears to be dominated

Figure 5. The solid line, a), represents the EPR X-band spectrum of the PANI-ES sample at 13 K. The dashed spectra b) and c) are decomposed spectra obtained as discussed in the legend of Fig 4.


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by two PC species which have different local spin dynamics, similarly to the R1 and R2 centers. It is important to note that such “doublet” spectral component exhibits also temperature dependent narrowing effects which can be used for an estimation of thermally induced activation processes that are related to these PC species. 3.4. Assignment of spectral components. The above analysis is one of the possible approaches to decompose the complex spectra of PANI-ES for gaining more detail information on the distribution of PCs in the sample. It is worth to note that the cumulative EPR spectrum of PANIES has the narrowest shape (Lorentz-type line with ∆pp = 0.068 mT) at about 20 K, see Fig. 1. Therefore, the above consideration supports the presence of various PCs with different degrees of electron delocalization along the polymer chain. The interchain interactions appear as a thermally activated process and contribute to the further narrowing of the PANI-ES spectrum. This behavior can be described by assuming the presence of typical triplet excited states, as generally discussed for conducting organic polymers.12 The triplet transition appears as a singlet spectrum if the exchange energy of this process is larger than the fine splitting, i.e. zero field splitting D for triplet (J >> D), as was discussed in detail previously.12 In order to more closely examine the nature of the distribution of the triplet states, the activation processes for paramagnetic species of PANI-HRPC-AOT were studied. These PCs exhibit a significantly larger splitting than the PCs of PANI-ES in the same low temperature region. Figure 6 shows the EPR spectrum of PANI-HRPC-AOT at 5 K


17


a) EPR Intensity (arb. units)

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b)

c)

330

335

340

345

350

355

360

Magnetic field (mT)

Figure 6. The solid line, a), represents the EPR X-band spectrum of the PANI-HRPC-AOT sample at 5 K. The second, b), and third, c), dashed spectra represent components of the complex detected spectrum obtained as discussed in the legend of Fig. 4. The double arrows denote major splitting between peaks in the b) spectrum denoted as RC1 spectral components and peak-to-peak linewidth in the c) spectrum denoted as RC2 spectral components. Besides the broad RC2 spectral component, an additional narrow peak component is present in the c) spectrum.

and the possible decomposed spectral components obtained by using the same procedure as described above for the PANI-ES sample. Therefore, the above considerations fully support our initial findings (Fig. 1) that the spectra of PANI-ES as well as of PANI-HRPC-AOT and PANITvL-AOT exhibit complex multi-component behavior with different temperature dependencies of the constituting components. It is possible to separate the first spectral component, RC1, representing a ‘’doublet’’ with splitting of ∆B as is denoted with the double arrow in the second spectrum b) in Fig. 6, and the second component, RC2, as a broad singlet with linewidth ∆pp which is also described with double arrow near the third spectrum c) in Fig. 6. The temperature


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dependencies of these spectral parameters, ∆B and ∆pp, were studied for the corresponding temperature interval for both PANI-HRPC-AOT and PANI-TvL-AOT samples (Fig. 7 and 8). As shown in Fig. 7, the obtained splitting parameter ∆B

3.0

2.5

∆Β (mT)

2.0

1.5

1.0

0.5

0.0

0

10

20

30

40

50

60

T (K)

Figure 7. Temperature dependence of the splitting parameter ∆B, for PANI-HRPC-AOT (triangles) and PANI-TvL-AOT (squares). The solid lines represent the best fit of the experimental data using equation (2).

1.8 1.6 1.4 1.2

∆pp (mT)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8 0.6 0.4 0.2 0

10

20

30

40

50

60

T (K)


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Figure 8. Temperature dependence of the linewidth ∆pp, for PANI-HRPC-AOT (triangles) and PANI-TvL-AOT (squares). The solid lines represent the best fit of the experimental data using equation (3).

decreases with increasing temperature and it could be described as typical merging of two lines due to a thermally activated process with correlation time, τ= τ0 exp(∆E/kT), in the slow rate limit13 ∆ ∆ 2 ! "/

(2)

Here, ∆B0 denotes splitting between the two spectral lines in absence of an activated process (τ → ∞) and γ represents the gyromagnetic ratio. In the fast rate limit, the line narrowing is expected as13 ∆## ∆## $

(3)

where ∆pp0 represents the spectrum peak-to-peak linewidth for (τ → 0) and c is a proportionality constant. Equations (2) and (3) can be used to estimate ∆E of the activation process related to the temperature dependence of the splitting of RC1 and the line narrowing of the broad singlet of RC2, respectively. The part of the linewidth of the EPR spectrum of PANI-ES which exhibited temperature dependence was obtained by subtraction of the narrowest linewidth, ∆pp0, in the monitored temperature interval. Two prominent temperature dependencies are clearly seen in Fig. 9. The linewidth decreases in the low temperature interval with increasing temperature (5 K - 20 K), while the linewidth exhibits a sharp increase with increasing temperature at higher temperatures. The narrowing behavior can be described by equation (3) with different activation


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processes (detected as different possible slopes shown in Fig. 9) within an approximate

-1

-2

ln(∆pp− 0.068) (mT)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3

-4

-5

-6

-7 0.00

0.05

0.10

0.15

0.20

0.25

-1

T (K )

Figure 9. . Plot of the natural logarithm of the temperature dependent contribution of ∆pp for PANI-ES vs. the inverse temperature. The narrowest line, i.e. ∆pp = 0.068 mT, from the all estimated linewidths was approximated as temperature independent and subtracted from all other measured lines in order to obtain temperature dependent contributions ((∆pp

0.068) mT). The

dashed lines represent linear fits (possible activation energies of activated process) in different temperature regions; their parameters are collected in Table 3.

range of ∆E (5.6 K - 90 K). The obtained energy range (∆E) is closely related to the energy interval of the so called ‘’narrow peak’’ of the distribution function of energy for singlet to triplet transitions as previously deduced14 in the description of SQUID magnetometry measurements of a portion of magnetic field dependent susceptibility of PANI films at low temperatures. Thus, the above analysis indicates that the inhomogeneously broadened EPR spectrum of PANI-ES at low temperatures contains distributions of spectral lines which originate from the distribution of triplet states.


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Table 3. Obtained parameters from the best fitted experimental data shown in Figures 7, 8 and 9 using equations (2), (3) and linear regression. The estimated errors for the fitted expression containing two parameters are less than 3%. Parmagnetic center

∆B0 (mT)

RC1 PANIHRPC-AOT

5.2 ±0.3

RC2 PANIHRPC-AOT RC1 PANITvL-AOT

∆pp0 (mT)

0.5 4.8±0.5

∆E (K)

τ0 (ns)

0.78±0.02

1.5 ±0.3

4.76 0.08±0.03

RC2 PANITvL-AOT

0.25

3.67

RC2 PANI-ES

0.068

~ (5.6 – 90)

1.7±0.5

The triplet spin exchange in organic radical salts was previously studied in detail by employing a similar approach of low temperature activation process described by equations (2) and (3) detected by EPR spectroscopy at low temperatures.15 In this approach the exchange frequency (2πν = τ-1, with τ given in the thermally activated process described with equations (2) and (3)) is related to the collision of pseudo-particles of spin S = 1 whose movement leads to motional narrowing of expected hyperfine and fine structure of the particles. It was shown that the singlettriplet energy difference, J, obtained from the concentration of the triplet excitations could be approximated with the obtained energy in the thermally activated process (J ≈ ∆E) of line narrowing, equations (2), (3). Generally, it is accepted15 that obtained ∆E is in the range ∆E < J for the slow rate limit process and ∆E ≥ J for the fast motional rate process. It is also important to note that when the excitation bandwidth is comparable to or less than kT, for the triplet excitation


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the fine structure splitting is expected to be temperature dependent. Indeed, the doublet structure of RC1 in PANI-HRPC-AOT could be related to a powder triplet spectrum13 with D ≈ ∆B0, (D = (3µ0/8π)g2β2 ). Although data on more precise spin delocalization within the radical type are not available, a very crude approximation based on the point dipole-dipole model leads to an average minimum distance r = 0.81± 0.02 nm between the paramagnetic centres for estimated ∆B0 = 5.2 ± 0.3 mT (Table 3). In accord with a model system which describes the distance between typical radicals of spin probes for a such short distance it was shown that this approximation deviates almost 40% giving shorter distance than the results derived from X-ray analysis.16 The model system of radical pairs16 which includes quantum chemical calculations for a more precise estimation of D and J parameters of radical pair can be used to scale the obtained experimental values (Table 2). In this case the obtained parameter for RC1 of PANI-HRPC-AOT, D ≈ ∆B0 = 5.2 ± 0.3 mT corresponds to a scaled value of distance r = 0.93 ± 0.02 nm between radical pairs. Moreover, in the same theoretical study16 on the exchange process between two unpaired electrons of spin probes, it was shown that for a distance of r = 0.93 nm one expects J ≈ 0.14 K. The same scaling procedure for RC1 of PANI-TvL-AOT shows that for the evaluated distance r = 0.96 nm one obtains J ≈ 0.10 K which is in a reasonable agreement with the experimentally measured J = 0.08 ± 0.03 K (Table 3.). In accord to X-ray diffraction data17 the minimum distances between polyaniline chains are in the range 0.57- 0.70 nm; therefore, the results obtained above could indicate the possible presence of enlarged distances between the polymer chains in PANI-HRPC-AOT and PANI-TvL-AOT, as compared to PANI-ES. Earlier, it was also shown that for the quasi-1D spin diffusion model of PANI-ES1a, intrachain exchange interactions, J

10 K, are expected around two orders of magnitude stronger than interchain

exchange interactions, J’

0.1 K. For RC2 one expects smaller average distances than for RC1,


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which is also associated with a stronger exchange interaction as can be seen in Table 3. It can be assumed (data in Table 3) that radical centers of the type RC2 with J

5 K are present in both

PANI-HRPC-AOT and PANI-ES samples. Furthermore, the linewidth analysis of RC2 in the PANI-ES sample (Figure 9 and Table 3) indicates additional presence of a population of triplet states with a distribution of significantly stronger exchange interactions. The small portion of cumulative spectra in Figure 5 and Figure 6, present as ‘’doublet’’ structure, could be also assigned to a small percentage of RC1 type of triplet spectrum in the PANI-ES sample. In addition, the temperature dependent contribution of (∆pp – 0.068 mT) for PANI-ES exhibits nearly exponential temperature dependence in the high temperature interval (20 K - 200 K) as shown in Fig. 9. The observed contribution to cumulative linewidth could be related to the part of contribution which originates from T1. 1b, 1j, 18 This exponential behavior (1/T1 ∝ exp(-∆E/T)) was detected previously in doped polyaniline and it was related to the Orbach type of process of spin lattice relaxation time.18 The activation energy obtained from Fig. 9 (∆E = 83.7 K) is ascribed to an excited energy level of the clusters of spins, which are strongly coupled to the lattice. As expected, the same contribution to linewidth for the PANI-HRPC-AOT and PANITvL-AOT samples is omitted since only narrowing of broad ∆pp can be detected in the same temperature interval (Fig.1). The above described magnetic properties of PANI-ES and of the two enzymatically synthesized PANI obtained in presence of AOT vesicles clearly indicate that difference in conductivity between these samples could be correlated with larger interchain distances in PANIHRPC-AOT and PANI-TvL-AOT than in PANI-ES. The spectral component which originates from the strong interchain interaction exhibits narrow linewidth and it is detected as dominant components in the cumulative EPR spectrum of PANI-ES. Detection of the same component in


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enzymatically synthesized PANI in presence of AOT vesicles was absent and only intensive broad components were present in the obtained EPR spectrum. 4. CONCLUSIONS In this work EPR spectroscopy was employed to study the magnetic properties of two samples of enzymatically synthesized PANI in the presence of AOT vesicles in comparison with a commercial PANI-ES sample. It was shown that although both PANI-HRPC-AOT and PANITvL-AOT had a higher spin concentration than PANI-ES, and that the measured conductivity at room temperature of these two samples,4c however, was much lower than the conductivity of the commercial sample. It seems that the AOT molecules still present in enzymatically synthesized PANI hindered intermolecular charge transport. Thus, we wanted to obtain some experimental evidence related to the spin concentration and the polaron dynamics which could support the above assumption on a reduced charge transport between the polymer chains. EPR line intensities and linewidths for all three polymer samples in a wide temperature range were therefore studied. The polaron concentration and the polaron interactions could be followed in a wide temperature interval from the obtained plots of χT vs. T. By applying a model of polaron bands, two well distinguished antiferromagnetically coupled bands could be used to describe these polymer samples. It is important to note that similar values of the parameters were used to describe the estimated polaron bands (Table 2); this indicates that the polaron dynamics detected in this case did not correlate with the conductivity process. Moreover, it was assumed that polarons with higher and lower J are generated for shorter and longer coplanar chains, respectively. In this context it is expected that the PANI-HRPC-AOT sample which exhibited a


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Page 26 of 32

band with the highest J should contain the shortest coplanar chains in comparison to other two polymers. The detected linewidths of EPR spectra clearly show differences between the enzymatically synthesized PANI and PANI-ES in the monitored temperature interval. Particularly, assignment of RC1 and RC2 as possible triplet states which originate from interchain exchange interactions and their different concentrations indicate the presence of different amounts of structural conformations in enzymatically synthesized PANI samples in comparison to the well known structure of PANI-ES. Thus, the presence of AOT in enzymatically prepared PANI samples seems to influence chain conformations and interchain distances. The above analysis of experimental data supports larger interchain distances in the enzymatically synthesized PANI samples obtained in the presence of AOT vesicles than in PANI-ES. Therefore, it is likely that the presence of remaining AOT in enzymatically synthesized PANI samples is the main reason for their reduced metallic character with quasi-3D domains and their reduced conductivity. It is also possible that small differences in the chemical structure of the enzymatically synthesized PANI samples, if compared to the commercial PANI-ES product, could be responsible for the observed differences in conductivity.4c Such possibility was discussed previously for chemically synthesized polyaniline with Fe3+ and H2O2,19 whereby the formation of overoxidized products led to a decrease in conductivity. In the case of PANI-TvLAOT, there is indeed evidence that the product obtained was overoxidized,4c while for PANIHRPC-AOT this was not the case.4b Therefore, we conclude that hindered interchain electron transport caused by remaining AOT molecules is probably the main reason for the low conductivity of the enzymatically synthesized PANI samples.


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ACKNOWLEDGMENT This work was supported by the Croatian Ministry of Science, Education, and Sports, Grant no. 098-0982915-2939 and by the Swiss National Science Foundation Grant no. 200020-130472.

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EPR Study of Polyaniline Synthesized Enzymatically in the Presence

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